Retention loss of reversed-phase chromatographic columns using 100% aqueous mobile phases from fundamental insights to best practice

This work deals with experimental investigations pertaining to the impact of chemical (electrolyte concentration from 0 to 100 mM, dissolved nitrogen gas from 0 to 6.7 × 10-4 M in water; surface chemistry including hexylphenyl, polyphenyl, C30, C18, and C8; surface coverage in C18-bonded chains from 1.5 to 3.5 µmol/m2; presence of surface dopant), physical (hydrostatic pressure of water from 50 to 500 bar; temperature from 27 ∘C to 75 ∘C), and structural parameters (average pore size from 50 Å to 400 Å; pore connectivity) on the dewetting kinetics of water from the hydrophobic mesopores of particles packed in RPLC columns. The results are explained from physico-chemical viewpoints involving intrusion and extrusion Laplace pressures, advancing and receding contact angles, surface tension of water, vapor pressure of water, 3D reconstruction of the actual mesoporous structure, pore connectivity, and the hysteresis in nitrogen adsorption and desorption isotherm onto reversed-phase chromatographic materials. A model of water dewetting consistent with the observations and the physical interpretations is then proposed. Finally, the most relevant practical solutions (pressurizing the column in absence of flow, pore size enlargement, using phenyl-bonded phase, polar embedded or surface doped C18-bonded phases, reducing the C18 surface coverage, doping the silica surface, lengthening of the alkyl-bonded chains, applying low temperatures, purging and degassing the mobile phase with helium gas) are suggested in order to eliminate or at least minimize the retention loss of RPLC columns when using fully aqueous mobile phases.